When microprocessors were introduced into the consumer market, the volume of microprocessors increased significantly. However, the desire to produce personal computing systems for the average consumer drove the cost and the associated profit margins down dramatically. In order to design within these cost constraints, all aspects of the personal computing system needed to be designed from a cost standpoint. One aspect of the personal computer that has been difficult to design from a cost minimization standpoint is the power supply. Although the performance of personal computers has increased and the cost of the microprocessors has gone down, the power supply requirements for these microprocessors has increased dramatically.
In early microprocessor designs, the microprocessor obtains power directly from the system's power supply assembly, with the system's power supply assembly providing a separate tap that could drive the microprocessor. With respect to these early microprocessor designs, load transitions were mild enough to allow operation with low cost point-load bulk storage capacitors, as well as by-pass capacitors to control ringing between the supply and ground. It was not uncommon for these early microprocessor designs to have a supply voltage tolerance of 10% of the nominal voltage level.
As microprocessors became more powerful, the microprocessor voltage was introduced at 3.3 volts with a 5% allowable tolerance. This required most manufacturers to provide DC-to-DC regulation on the board itself. Both linear and switching regulation solutions were utilized successfully. With single processor designs, the linear regulation method was less costly, but did result in a higher thermal dissipation due to various loads. These were still controllable through the design of the supply itself. When multiple processors were utilized on the single processor board, switching regulation was typically utilized as the power supply of choice due to the thermal considerations, i.e., switching regulators are much more energy efficient. However, from a design standpoint, a linear regulator has an inherently better dynamic response than a switching regulator, whereas the switching regulator has superior power conversion efficiencies which equate to thermal management within a power supply design. When utilizing a switching regulator design to accommodate the thermal necessities of the design, power supply designers typically utilize capacitors having a very low Equivalent Series Resistance (ESR). However, these capacitors are fairly expensive and are not consistent with the cost objectives for the power supply design in consumer personal computing systems.
One system for solving the above problems is disclosed in U.S. Pat. No. 4,893,228. This system utilizes a switching pre-regulator and a linear regulator. The pre-regulator is utilized to lower the primary voltage prior to the linear regulation operation such that the voltage across the pass element in the linear regulator is not the difference between the primary voltage and the regulated voltage output.